1). Field of the Invention
This invention relates to optical networks, and more particularly to optical network devices used in optical networks.
2). Discussion of Related Art
Optical networks consist of electrically powered optical nodes which communicate with each other primarily using optical signals. The optical signals are usually confined to optical fibers which physically interconnect the optical nodes.
The optical nodes can possess a wide range of capabilities which at a high level include optical functionality (such as optical amplification, optical switching, optical multiplexing, etc.), electrical functionality (such as power supply, node control and monitor, electrical data switching, etc.), and opto-electronic functions (such as the conversion between signals in the optical domain to the electrical domain and vice versa). In high traffic optical networks, multiple wavelengths can be combined in a single fiber. This process, called wavelength division multiplexing (WDM), allows a number of different signals to be independently transmitted across the same physical medium.
In such an optical network, for example, electrical data traffic can be processed and used to modulate a light source resulting in an optically modulated signal. This process is termed an electrical-to-optical (EO) conversion. Typically, the modulation rates in dense wavelength division multiplexing (DWDM) systems are concentrated around current telecommunication and data standards of nominal 2.5 gigabit per second (Gbps) and 10 Gbps class signals.
This signal is generated at one optical node and may be combined with one or more wavelengths onto a single fiber in a wavelength division multiplexing process. The signal can be optically amplified either on its own or as part of the wavelength division multiplex aggregate. The signal may then undergo separation via wavelength division multiplexing processes, optical switching, optical amplification/regeneration and optical combination at one or more intermediate nodes until it is terminated at its destination node. At intermediate nodes, or destination nodes, the signal is separated out and undergoes optical-to-electrical (OE) conversion and subsequent processing to recover the original data traffic and electrical format. Current technologies are such that different receivers are commonly used for 10 Gbps class signals and 2.5 Gbps class signals.
The node where the signal undergoes the EO conversion may be referred to as the “ingress” or originating node, the node where the signal undergoes the OE conversion may be referred to as the “egress” or terminating node, and the node that the signal enters and leaves an optical format may be referred to as pass-through nodes. Ingress and egress nodes may also be referred to as “access” nodes.
During transmission across an optical network, the dated traffic may be different as it leaves the network compared to when it enters the network. There are many reasons for this. During transmission, the optical signal quality is degraded such that at egress errors are made in the OE process. The degradation of the optical signal quality may take the form of noise added to the signal and/or pulse shaped distortion. Both noise and distortion can arise at multiple points along the optical transmission path. Normally, these effects are calculated and accounted for. However, both noise and distortion can change with environmental conditions and as devices age. If these changes are sufficient, the number of errors made in the OE process, also known as the number of errored bits per second, or “bit error rate” (BER), will increase and the network can become unusable. Under such conditions, it may be very difficult to determine where in the network the problem occurred.
One of the causes of signal distortion is known as chromatic dispersion. Chromatic dispersion is a property that results in different parts of the optical signal traveling at different speeds. While present to some degree in most optical components, chromatic dispersion of the fiber itself is usually of the most concern. At the far end of the fiber, the different parts of the signal add up in a way that is different from when it was launched, resulting in a distortion of the false shape relative to the shape of the starting pulse. The amount of distortion depends on the fiber type, the length of the fiber, and the characteristics of the source light. The effects of chromatic dispersion are most harmful at higher data rates.
The most commonly used way to combat the effects of chromatic dispersion is to send the signal through a device that has an equivalent amount of dispersion, but with the opposite sign. The effect of the opposite sign is to undo the distortion that was introduced as a result of the transmission through the fiber thereby compensating for the chromatic dispersion. The usual device used to accomplish this compensation is known as a dispersion compensating fiber. The length of the dispersion compensating fiber determines the amount of dispersion for which it compensates and is typically selected to be matched for a particular fiber length. Such a particular fiber device is called a “Dispersion Compensating Fiber (DCF)” module.
Recently, other devices have entered the market to solve the same problem. One such device is a tunable dispersion compensation module (TDCM), which has a number of advantages, the most important being that because it is tunable, one device can be used for a wide range of fiber lengths. However, it is still difficult to determine how much dispersion needs to be compensated for and to confirm that the device has been set properly. If the device has not been set properly, or degrades over time, the optical signal will be significantly distorted.